This invention relates to optical systems in general and more particularly to an improved unobscured anastigmatic telescope which is constructed with all spherical surfaces.
Most telescope systems presently in use, constructed from mirrors, require at least one aspheric surface. When these telescopes are used in an unobstructed, off axis version, expense becomes extremely high because of the need to make a full aspheric surface in order to obtain a section thereof for use in the telescope. Furthermore, the use of aspheric surfaces creates alignment problems.
There has, however, been developed, 80 years or so ago, a system known as the Schwarzschild system. This comprises two spherical mirrors, one concave and one convex. The parallel light to be imaged enters through a central aperture in the concave mirror, it is reflected from the convex mirror to the concave mirror which then forms an image. The system is well corrected for spherical aberration, coma and astigmatism. However, the primary drawback of the system and the reason why it has not gained wide acceptance is that the concave spherical mirror must be much larger than the entrance aperture.
There is a need for improved telescopes, particular telescopes which are capable of providing a strip or a ring field. Such telescopes, for example, are used in spacecraft for scanning purposes, the scanned information then being transmitted to the Earth and reconstructed into an image over a wide field of view. Such systems may also find use in generating ring field images for various purposes.
A ring field system which is useful for forming in accurate micro detail an image of an object with high resolution utilizing only spherical mirrors is disclosed in U.S. Pat. No. 3,748,015. This is also a system which is free from spherical aberration, coma and distortion. This, however, is a system which requires an object to be imaged, i.e., it is not in the nature of a telescope.